EP0566435A1 - Trickle heat-exchanger and an air destillation comprising such a heat-exchanger - Google Patents

Abstract

The nitrogen condensation passages (19) are closed at the top by a small bar (22), above which a liquid oxygen distribution compartment (23), closed at its upper end, is defined. This compartment contains a perforated bar (27) halfway up and, underneath this bar, packing (28) with cracks having horizontal generatrices. The liquid oxygen is introduced sideways into these compartments, predistributed by the orifices (33) of the bar, finely distributed by the packing (28), and passes into the adjacent oxygen vaporization passages (18) which are open at the top and bottom, through a horizontal slot (34) situated just above the small bar (22). Application to main vaporizer/condensers of double-column air distillation installations. <IMAGE>

Description

The present invention relates to a liquid flow heat exchanger for vaporizing a liquid by heat exchange with a second fluid, of the type comprising a parallelepipedal body formed by an assembly of parallel vertical plates defining between them a multitude of flat passages distributed in a set of vaporization passages and in a set of heating passages, each passage, in its current heat exchange part, containing a spacer wave with vertical generators, means for distributing the liquid being provided at the upper end of the exchanger for distributing the liquid over the entire length of the vaporization passages, and means being provided for sending the second fluid into the heating passages. It applies in particular to air distillation installations.

In air distillation systems of the double column type, the liquid oxygen which is in the bottom of the low pressure column is vaporized by heat exchange with the nitrogen gas at the head of the medium pressure column. For a given operating pressure of the low pressure column, the temperature difference between oxygen and nitrogen made necessary by the structure of the heat exchanger imposes the operating pressure of the medium pressure column. It is therefore desirable that this temperature difference is as small as possible, in order to minimize the expenses linked to the compression of the air to be treated injected into the medium pressure column.

To achieve this goal by benefiting from the very advantageous technology of brazed plate heat exchangers, EP-A-0 130 122 in the name of the Applicant has proposed a particularly efficient mode of distribution of liquid oxygen.

However, whatever the distribution method adopted, current technology has certain limits. These are due to the fact that, while the liquid oxygen is at a pressure which is only slightly higher than atmospheric pressure, the gaseous oxygen resulting from the vaporization must evacuate itself from the 'exchanger. The pressure drop in the path of the gaseous oxygen must therefore be very low. In all known solutions, this constraint limits the height of the exchanger, and more generally its performance.

The object of the invention is to make it possible to increase the height of such a heat exchanger or, at a given height, to reduce the pressure drop of the flow of vaporized oxygen. Actually, it relates to a heat exchanger of the aforementioned type, characterized in that said distribution means are arranged in closed compartments at their upper end and each located above a heating passage, from which it is separated. by a horizontal bar, in that a horizontal slot, extending over the entire length of the exchanger, just above the bar, puts the lower part of the compartment in free communication with an adjacent vaporization passage, and in that the spray passages are open at their upper and lower ends, over their entire length, and contain at most one wave-spacer with vertical generators at any point of their height.

• - les passages de vaporisation sont dépourvus de toute onde-entretoise en regard des fentes;
• - la surface supérieure de la barrette est inclinée latéralement vers la fente;
• - les moyens de distribution du liquide comprennent, d'une part, une barre horizontale s'étendant sur toute la longueur de chaque compartiment, à un niveau intermédiaire de celui-ci, cette barre ayant une épaisseur égale à l'espacement mutuel des plaques et comportant des ouvertures de prédistribution du liquide, et d'autre part, au-dessous de cette barre, un garnissage de distribution fine du liquide sur toute la longueur horizontale du compartiment;
• - lesdites ouvertures forment une rangée horizontale de trous équidistants les uns des autres;
• - la barre comporte sur une face verticale un ou plusieurs évidements arrière fermés en bas et ouverts vers le haut et sur son autre face verticale un ou plusieurs évidements avant ouverts vers le bas et fermés vers le haut, et en ce que lesdites ouvertures sont ménagées à travers une paroi verticale commune aux évidements avant et arrière;
• - la barre comporte plusieurs évidements arrière espacés les uns des autres, et plusieurs évidements avant espacés les uns des autres;
• - les évidements avant ont une forme évasée vers le bas;
• - le garnissage est une onde à génératrices horizontales dont les flancs sont munis de crevés;
• - le garnissage est espacé de la surface supérieure de la barrette;
• - l'échangeur comporte une boîte d'entrée latérale de liquide dans lesdits compartiments, le point bas de cette boîte se trouvant au-dessous du point bas de la fenêtre d'entrée de ces compartiments.

According to other characteristics:

• - the spray passages are devoid of any wave-spacer facing the slots;
• - The upper surface of the bar is inclined laterally towards the slot;
• - The liquid distribution means comprise, on the one hand, a horizontal bar extending over the entire length of each compartment, at an intermediate level thereof, this bar having a thickness equal to the mutual spacing of the plates and comprising liquid predistribution openings, and on the other hand, below this bar, a lining for fine distribution of the liquid over the entire horizontal length of the compartment;
• - Said openings form a horizontal row of holes equidistant from each other;
• - The bar comprises on one vertical face one or more rear recesses closed at the bottom and open at the top and on its other vertical face one or more front recesses open at the bottom and closed at the top, and in that said openings are provided through a vertical wall common to the front and rear recesses;
• - The bar has several rear recesses spaced from each other, and several front recesses spaced from each other;
• - the front recesses have a flared shape down;
• - the lining is a wave with horizontal generatrices whose sides are provided with punctures;
• - the lining is spaced from the upper surface of the bar;
• - The exchanger includes a lateral liquid inlet box in said compartments, the bottom point of this box being below the bottom point of the inlet window of these compartments.

The invention also relates to an air separation installation by distillation, of the type comprising a first distillation column operating under a relatively high pressure, a second distillation column operating under a relatively low pressure, and a permanent heat exchanger. as long as putting the liquid oxygen in the tank of the second column in heat exchange relation with the nitrogen gas at the top of the first column, characterized in that the heat exchanger is as defined above, and in that the instal lation comprises supply means for supplying liquid oxygen to said liquid distribution means, and means for supplying the heating passages with nitrogen gas.

• - la Figure 1 est un schéma partiel d'une installation de distillation d'air conforme à l'invention;
• - la Figure 2 représente en coupe verticale, à plus grande échelle, la région Il de la Figure 1, la coupe étant prise suivant la ligne II-II de la Figure 4;
• - la Figure 3 est une vue partielle en plan prise suivant la flèche III de la Figure 2;
• - la Figure 4 est une vue prise en coupe suivant la ligne IV IV de la Figure 2; et
• - la Figure 5 est une vue analogue d'une variante.

Examples of implementation of the invention will now be described with reference to the accompanying drawings. In these drawings:

• - Figure 1 is a partial diagram of an air distillation installation according to the invention;
• - Figure 2 shows in vertical section, on a larger scale, the region II of Figure 1, the section being taken along line II-II of Figure 4;
• - Figure 3 is a partial plan view taken along arrow III of Figure 2;
• - Figure 4 is a sectional view taken along line IV IV of Figure 2; and
• - Figure 5 is a similar view of a variant.

Figure 1 illustrates a possibility of installing an oxygen-nitrogen heat exchanger in an air distillation installation of the double column type. This installation comprises a medium pressure column 1 at the bottom of which the air to be treated is injected, under a pressure of the order of 6 bars absolute. The oxygen-enriched liquid which is collected in the tank of column 1 is sent under reflux in the middle of the height of a second column (not shown), called the low pressure column, which operates slightly above atmospheric pressure. The nitrogen gas which is at the head of column 1 is brought into an indirect heat exchange relationship with the liquid oxygen collected in the bottom of the low pressure column; the resulting condensed nitrogen serves as reflux in column 1 and in the low pressure column, while the resulting vaporized oxygen is returned to the bottom of the low pressure column.

The two distillation columns can in particular be of the packed type, which also contributes to the energy gain by lowering the operating pressure of the installation, which is that of column 1.

The heat exchange between oxygen and nitrogen takes place in an exchanger 2 which is mounted above the column 1, while the low pressure column is juxtaposed to the latter.

The exchanger 2 consists of a sealed envelope 3, the main height of which contains a set of parallel plates 4 of rectangular aluminum shape, with a length of the order of 1 to 1.5 m and a height of the order of 3 to 7 m, between which waves also made of aluminum are fixed by brazing.

A space under a pressure slightly higher than that of the low pressure column (for example of the order of 1.4 bar), located at the upper end of the plates 4, opposite one of their vertical sections , contains a liquid oxygen bath 5 supplied with rain by a line 6 coming from the tank of the low pressure column and provided with a pump (not shown). The latter can be controlled by a regulator of the bath level 5, or, alternatively, by a flow regulator. At the top of the exchanger 2, the casing 3 forms a dome 7 which contains the bath 5. From this dome leaves a pipe 8 for returning to the bottom of the low pressure column of the vaporized oxygen coming from the bath 5, resulting from the heat inputs at the level of the pump and the pipes, and part of the oxygen vaporized in the exchanger 2.

The set of plates 4 is supplied at its upper part with nitrogen gas at 6 bars by a horizontal supply box 9, located under the bath 5, which communicates by a pipe 10 with the head of the medium pressure column. The evacuation of the condensed nitrogen is carried out at the base of the plates 4 by a horizontal collecting box 11 which communicates by a pipe 12 with a guarded channel 13 disposed at the head of the column 1. On the box 11 is stuck a pipe 14 for the discharge of uncondensable rare gases.

A pipe 15 connects the tank of the low pressure column to the space located in the casing 3, below the plates 4. This pipe penetrates vertically into this space through the bottom point of the casing 3, and its upper end is surmounted by a conical deflector 16. From the bottom of the casing 3 also leaves a pipe 17 intended to return to the bottom of the low pressure column the excess liquid oxygen.

The structure of the active part of the exchanger 2, that is to say of the set of plates 4, will now be described with reference to FIGS. 2 to 4.

In this region, the exchanger has a parallelepiped shape, and the casing 3 is defined by the edges of the plates 4 and by spacer bars which close the passages that these plates define, except at the inlet and outlet locations for the fluids. . The plates 4 define a multitude of passages intended alternately for the flow of oxygen (passages 18) and for the flow of nitrogen (passages 19). Over most of their height, the passages 18 and 19 each contain a spacer wave 20 consisting of a corrugated perforated aluminum sheet with vertical generators.

The waves 20 of the nitrogen passages end, at the top as well as the bottom, before the waves 20 of the oxygen passages. At the bottom of the plates 4, these waves of the passages 19 are extended by oblique waves of nitrogen collection (not shown) which lead to the inlet of the manifold 11. At their upper end, these same waves 19 are extended by oblique waves 21 of nitrogen distribution which open, through a side window 21A of the exchanger, at the outlet of the feed box 9. Above the waves 21, the nitrogen passages 19 are closed by horizontal bars 22. Other horizontal bars (not shown) close the lower end of the nitrogen passages below the collection zones of nitrogen. Above the bars 22, each nitrogen passage is extended by a compartment 23 for distributing liquid oxygen closed at the upper end of the exchanger by a horizontal bar 24. The compartment 23 contains, from top to bottom: an oblique spacer wave 25 (or, as a variant, a perforated wave with horizontal generators) for rough distribution of liquid oxygen over the entire length of the compartment, this wave opening out laterally, via a side window 26 of the exchanger, in bath 5 (Figure 2); a perforated bar 27 for predistribution of liquid oxygen; and a lining 28 for fine distribution of liquid oxygen. A free space 29 is provided between this lining and the upper surface of the bar 22.

The bar 27 is machined from a rectangular blank whose thickness is equal to the spacing of the plates 4, ie of the order of 5 to 15 mm, and whose length is equal to that of these plates. In one of its large faces are machined a series of rear recesses (considering Figure 2) 30 in U, open upwards, and in its other face are machined a series of front recesses 31 roughly in semicircle, open down. Each recess 31 is located longitudinally opposite a recess 30 and overlaps the latter in height, so that there is, approximately halfway through the thickness of the bar (FIG. 4), a thin vertical wall 32 common to the two recesses. This wall is pierced with a circular hole 33. The holes 33 are distributed at regular intervals along the bar 27.

The lining 28 is constituted by a wave with horizontal generators (so-called "hard way" arrangement with respect to the flow of liquid oxygen) not perforated but of the "serrated" type. This means that at regular intervals, each horizontal or pseudo-horizontal facet of the wave is provided with a flat offset upward by a quarter of a wave step. The width of the punctures, measured along a generatrix of the wave, is of the same order as the distance which separates each of them from the two adjacent punctures located on the same facet.

The oxygen vaporization passages 18 are open at their upper and lower ends. They contain the wave 20 from the lower end to the level of the bars 22, are devoid of any wave opposite the space 29, then, from the upper level of this space 29 to their upper end, they contain another spacer wave 20A similar to wave 20 but not larger. The region of each wave-free passage 18 communicates freely with the space 29 of an adjacent passage 19 through a horizontal slot 34 of the same height extending over the entire length of the exchanger. Thus, one plate out of two is continuous over the entire extent of the exchanger, while one plate out of two is in fact made up of a rectangular plate 4A which extends upwards only up to the bar 22, and a rectangular plate 4B which delimits the compartment 23 for dispensing liquid oxygen. The upper surface 35 of the bar 22 is inclined laterally so as to descend from the adjacent plate 4 to the upper edge of the facing plate 4A. By virtue of a recess of the bar 22, this surface extends slightly beyond the face of the plate 4A which delimits the passage 18.

In operation, the liquid oxygen bath 5 is maintained at an approximately constant level, without exceeding the upper face of a vertical plate 5A welded on the exchanger above the windows 26. Thus, the liquid oxygen penetrates laterally in the compartments 23, by one of their ends through the windows 26. Simultaneously, the nitrogen gas at 6 absolute bars enters the upper part of the passages 19, by one end of these passages, via the box 9 and the waves distribution 21.

The liquid oxygen thus forms a liquid column of practically uniform height above all the holes 33. It is predistributed over the entire length of the passages 18 into a number of jets 36 by these holes 33, then falls freely on the packing 28, which, by its constitution and its arrangement, ensures a fine distribution of the liquid oxygen all along the passages 18. The liquid oxygen therefore falls uniformly on the inclined surface 35 of the bars 22, then pours through the slots 34 in the passages 18.

A film of liquid oxygen thus flows over all the metallic surfaces contained in the passages 18, that is to say on the plates 4 and 4A and on the waves 20, and it partially vaporizes by indirect heat exchange with the nitrogen in the course of condensation from top to bottom in the alternate passages 19.

As indicated above, the passages 18 are not only open upwards and downwards, but also free as much as possible, over their entire height, of obstacles to the flow of gaseous oxygen. Indeed, at any point of their height, these passages are either empty (opposite the slot 34), or provided with a single wave 20, 20A with vertical generators and with relatively large pitch. Wave 20 improves heat exchange with nitrogen by fin effect, while wave 20 Asert only spacer and can even possibly be partially eliminated.

It follows from this that part of the vaporized oxygen can leave the exchanger from above and therefore add, in the upper dome 7, to the evaporations of the bath 5 (Figure 1), the rest of the oxygen vaporized leaving the heat exchanger from below at the same time as the liquid oxygen in excess then being evacuated via the pipe 15. The two outlet paths of the vaporized oxygen are traversed by a reduced gas flow, and each path further imposes a minimum pressure drop on the flow of this gas. Finally, the height of the exchanger can be increased.

It should be noted that thanks to the structure of the bars 27, the holes 33 have a horizontal axis and there is a dead end 37 on the rear face of the bar, below these holes. Any solid impurities contained in the liquid oxygen can thus be deposited in these dead ends, which protects the holes 33 against the risks of clogging.

Likewise, the configuration of the envelope 3 in the region of the liquid oxygen bath 5 forms a cul-de-sac 38 adjacent to and situated under the inlet windows 26, which allows the most solid impurities large to settle in this cul-de-sac, leaving the supply line 6, as indicated at 39 in Figure 2.

If, in a particular application, this settling is considered sufficient to avoid any risk of plugging of the holes 33, we can use the variant of Figure 5. This differs from the previous one only by the simplified construction of the bar 27, which is a simple rectangular section bar provided at regular intervals with holes 33 with a vertical axis. These holes may have an enlarged diameter over most of their height from the bottom, as explained in the aforementioned EP-A-0 130 122.

Claims (12)

1 - Liquid flow heat exchanger for vaporizing a liquid by heat exchange with a second fluid, of the type comprising a parallelepipedic body formed by an assembly of parallel vertical plates (4) defining between them a multitude of flat passages (18, 19) distributed in a set of vaporization passages (18), and in a set of heating passages (19), each passage, in its current heat exchange part, containing a wave-spacer (20) with vertical generators, liquid distribution means being provided at the upper end of the exchanger (2) for distributing the liquid over the entire length of the vaporization passages (18), and means (9) being provided for sending the second fluid in the heating passages (19), characterized in that said distribution means are arranged in compartments (23) closed at their upper end and each located above d '' a heating passage (19), from which it is separated by a horizontal strip (22), in that a horizontal slot (34), extending over the entire length of the exchanger, just above the bar (22), puts the lower part of the compartment (23) in free communication with an adjacent spray passage (18), and in that the spray passages (18) are open at their upper and lower ends, over any their length, and contain at most one wave-spacer (20A) with vertical generators at any point of their height. 2 - Exchanger according to claim 1, characterized in that the vaporization passages (18) are devoid of any wave-spacer facing the slots (34). 3 - Exchanger according to claim 1 or 2, characterized in that the upper surface (35) of the bar (22) is inclined laterally towards the slot (34). 4 - Exchanger according to any one of claims 1 to 3, characterized in that the liquid distribution means comprise, on the one hand, a horizontal bar (27) extending over the entire length of each compartment, to a intermediate level thereof, this bar having a thickness equal to the mutual spacing of the plates (4) and comprising openings (33) for predistribution of the liquid, and on the other hand, below this bar, a lining (28) for fine distribution of the liquid over the entire horizontal length of the compartment (23). 5 - Exchanger according to claim 4, characterized in that said openings (33) form a horizontal row of holes equidistant from each other. 6 - Exchanger according to claim 4 or 5, characterized in that the bar (27) has on one vertical face one or more rear recesses (30) closed at the bottom and open upwards and on its other vertical face one or more recesses front (31) open downward and closed upward, and in that said openings (33) are formed through a vertical wall (32) common to the front and rear recesses. 7 - Exchanger according to claim 6, characterized in that the bar (27) has several rear recesses (30) spaced from each other, and several front recesses (31) spaced from each other. 8 - Exchanger according to claim 7, characterized in that the front recesses (31) have a flared shape downwards. 9 - Exchanger according to any one of claims 4 to 8, characterized in that the lining (28) is a wave with horizontal generators whose sides are provided with punctures. 10 - Exchanger according to any one of claims 4 to 9, characterized in that the lining (28) is spaced from the upper surface (35) of the bar (22). 11 - Exchanger according to any one of claims 1 to 10, characterized in that it comprises a side liquid inlet box in said compartments (23), the low point of this box is located below the low point of the entry window (26) for these compartments. 12 - Installation for separation of air by distillation, of the type comprising a first distillation column (1) operating under a relatively high pressure, a second distillation column operating under a relatively low pressure, and a heat exchanger (2) allowing to put the liquid oxygen in the tank of the second column in heat exchange relation with the nitrogen gas at the top of the first column, characterized in that the heat exchanger conforms to any one of claims 1 to 11, and in that the installation comprises supply means (6) for supplying liquid oxygen to said liquid distribution means, and means (9) for supplying the heating passages with nitrogen gas.

Images